‘CHARMed, I’m sure’

The first in a regular series of short features on the latest in cutting-edge audio research

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SHARES

Rescuing important old recordings from obscurity and making them available to the wider public is one of the goals of CHARM, the AHRC Research Centre for the History and Analysis of Recorded Music. One recent CHARM-hosted initiative has been ‘Musicians of Britain and Ireland,’ a JISC-funded archive project at King’s College London that restored 2000 sides of 78 rpm discs from the King’s Sound Archive and made them available as streamed MP3s or high quality downloads. These recordings mainly represent performances of British and Irish artists between 1900 and 1950, particularly those who fell by the wayside after the merger of the Gramophone Company and Columbia to form EMI in 1931. The mastering decisions involved in transferring files from 78 rpm discs are many, including whether to leave them in their raw state, noise and all, or whether to attempt to improve the quality by using noise reduction or EQ. Andrew Halifax (pictured), CHARM’s transfer engineer, says it’s about reproducing the original audio content with as much fidelity as possible in a way that is accessible to those without specialist skills or expertise. Because the main purpose is to offer material in a form suitable for study by musicians and musicologists, some modification of the raw material is considered acceptable in the interests of uncovering the music. Halifax starts by selecting the best copy from the discs available, as some of them are worn, cracked or even broken. The least worn disc might be chipped at the edge, so he has to decide whether to use it and edit in the beginning from another version, or tolerate the surface noise of a poorer copy. A Keith Monks machine is used to give the disc a wash and brush-up using double distilled water and a vacuum pump to suck away the mess. Then there is the thorny problem of which stylus to use. Small differences in styles size can make quite a big change to the distortion and noise levels, as well as to the high frequency response, for example. There’s also the tracking weight to consider – Halifax points out that you often only get the best sonic reproduction of the content along with an excellent reproduction of the surface noise. Replay pitch control is rarely straightforward because early discs were recorded at all sorts of speeds from around 68 to as much as 90 rpm, so it’s necessary to resort to whatever evidence can be found of the correct pitch of the music. An EMT 948 turntable with a special speed control is used here. Rather than immediately processing the transfer, a ‘flat’ version is made with no EQ or noise reduction. This one is archived so that there’s always a version on file that has not been interfered with, and each side of each disc is stored as a separate file. EQ can then be applied to correct for the recording characteristic, which is usually done by ear using a custom equaliser because standard curves were often varied by cutting engineers. CEDAR’s decrackler and declicker are used to clean up interfering noises, although broadband noise is usually left alone. Lossless FLAC encoding is used to create high quality downloadable files and MP3s are created for ordinary streaming. XML files store metadata that describes the entire transfer process. Listen and be CHARMed at http://www.charm.kcl.ac.uk/sound/sound_search.html